Question

Transistion of exarch vascular bundle in root to endarch in stem in hypocotyl explain

Answer 1

The change in orientation of vascular bundles from exarch radial type in root to endarch conjoint collateral type in stem is called as root-stem transition. This occurs in the hypocotyl region. According to Eames and MacDaniels (1947), there are 3 types of root stem transitions in dicot and single type in monocot. Type 1 is when the phloem remains in the same place, whereas the xylem halves are inverted and become conjoint and endarch in stem; eg: Mirabilis, Fumaria. Type 2 has twice the number of bundles in stem than root. Phloem, in its upstream, is constricted and this constriction eventually leads to the splitting of phloem. The two xylem strands also split into four and are inverted same as in type 1. These join to phloem to become conjoint; eg: Cucrubita, Acer. In type 3, number of vascular bundles in stem remains same as number of phloem in root. Instead of being radial, they become oriented conjointly. Two phloem strands splits into four, swings laterally to join xylem on the outside and fuses to again form two phloem strands; eg, Lathyrus, Phoenix. In type 4, a reductional division occurs where the number of vascular bundles in stem becomes half the number of phloem strands in root. The four strands of phloem fuse during the transition to become two strands. Xylem divides into two diagonal pairs which ventually join together to form single Co joint vascular bundle in stem which has five vascular components; eg. Anemarrhena ( Fam: Lilliaceae)